Possible Clues:

• A line like a contour, seen on some weather maps
• I thus exclude an indication of pressure
• I cry a river, which indicates constant pressure
• Line on chart is old watering-hole
• Line on a map
• Sloppy Boris has a way to deal with pressure
• Isogram
• Descriptive line from Boris, upset about article?
• Line on map represents round watering hole
• Line connecting points of equal pressure